A sensor inductor (1) for magnetic fields, particularly of the saturating-core type, has at least one inductor coil (10) which is wound as a planar spiral about a core (20). More preferably a plurality of planar spirals (10a-10d) are stacked along the axis (50) of the core. A pair of adjacent coils (10a:10b) are wound as spirals of opposite sense with the inner ends (32a: 32b) connected together by bridges (44). Two or three such pairs may be used. The plural coils (10a-10d) and the central core (20) may be fabricated using planar selective deposition techniques known from the semiconductor industry. The magnetic core (50) may be a separate entity inserted into an axial hole formed through the stack of coils. A preferred core material is permalloy. Turns width (w), spacing (s) and thickness (t) of the spiral coils is discussed together with the axial spacing (d) between coils and core diameter (Ζ)